var tension = 1;
var poly = document.querySelector("polyline");
var path = document.querySelector("path");
var points = [
600,200,
700,350,
100,350,
200,100,
300,350,
400,150,
500,350,
600,200,
700,350,
100,350,
200,100
];
poly.setAttribute("points", points);
path.setAttribute("d", solve(points, tension));
function solve(data, k) {
if (k == null) k = 1;
var size = data.length;
var last = size - 4;
var path = "M" + [data[2], data[3]];
for (var i = 2; i < size - 4; i +=2) {
var x0 = i ? data[i - 2] : data[0];
var y0 = i ? data[i - 1] : data[1];
var x1 = data[i + 0];
var y1 = data[i + 1];
var x2 = data[i + 2];
var y2 = data[i + 3];
var x3 = i !== last ? data[i + 4] : x2;
var y3 = i !== last ? data[i + 5] : y2;
var cp1x = x1 + (x2 - x0) / 6 * k;
var cp1y = y1 + (y2 - y0) / 6 * k;
var cp2x = x2 - (x3 - x1) / 6 * k;
var cp2y = y2 - (y3 - y1) / 6 * k;
path += "C" + [cp1x, cp1y, cp2x, cp2y, x2, y2];
}
return path;
}
body {
background: #222;
}
svg {
position: absolute;
width: 100%;
height: 100%;
}
polyline {
fill: none;
stroke: #888;
stroke-width: 1;
}
path {
fill: none;
stroke: cornflowerblue;
stroke-width: 2;
}
<svg>
<polyline />
<path />
</svg>